Air carrier flight crew reported receiving a GPWS terrain warning as well as a low altitude alert from ATC on approach to DEN following a miscommunication with the Controller.
Synopsis
Air carrier flight crew reported receiving a GPWS terrain warning as well as a low altitude alert from ATC on approach to DEN following a miscommunication with the Controller.
Narrative
We were on arrival into DEN on the AALLE3 arrival; just passing the fix 'DOGGG' at 11;000; and Approach Control gave us descend and maintain 8000; we responded with the same instruction and descended to 8000. While we were descending Approach said there was wind hear advisors in effect and that there had been multiple go arounds. The frequency was congested and other aircraft were being called by Approach Control to speed up for spacing and aircraft were responding with unable. Approach Control responded that she was trying to space out all aircraft to give everyone a chance for approach to land. Approach Control called us and instructed us to descend and maintain 6;500 feet and my FO (First Officer) repeated the clearance; at this time the approach Controller 'did not' reply with an altitude correction or change. Approach Control instructed us to turn heading 270 and slow to 190 knots; my FO repeated the instruction. I increased my descent to 2;000 feet per minute and made the right turn to 270; however my speed was increasing to 220 so I extended the spoilers to slow to 190. I reached up to slow my descent rate; and I realized autopilot did not capture 6;500 feet; I looked down and my speed was decreasing to 185-190 knots. The low speed indicator increased with the stall margin indicator; and we got the terrain warning once and I immediately and simultaneously; disconnected autopilot and got a momentary stick shaker. I stowed the spoilers; added max thrust; and leveled for a split second and as speed increased with no drag I initiated a climb up to 7;500 feet. I advised my FO to tell Approach we were correcting; Approach was calling to confirm our altitude and advise us that we should be at 7;500 feet and they received a low altitude alert for us. My FO advised approach that we were climbing to 7;500 and Approach responded with everyone watch their altitudes; and advised us to turn heading 325; and maintain 150 knots until FRONZ and cleared us for the visual to 35R. My FO repeated the clearance. The winds were extremely gusty and our turn was wider with the strong winds and windshear. At this time the intercept heading came to late from approach; and I overshot the approach course and I was already in a turn to re-intercept the approach course. We intercepted the course; and were configuring to be stabilized by the final approach fix FRONZ. We were fully configured and stable by FRONZ and approach instructed us to contact Tower; Tower cleared us to land. After being cleared to land; we encountered windshear caution; but continued and then Tower called and advised us to expect a loss of 30 knots at 150 feet; we continued and made a safe landing on Runway 35R.I believe the cause of this event was due to Approach giving us an altitude of 6;500 feet; a heading instruction and speed reduction instruction. I believe the terrain warning which alerted Approach; was due to an aggressive descent rate with autopilot connected at a low altitude. I believe the stick shaker was caused by the fully extended spoilers which I had extended to slow to 190 knots per approaches speed restriction and my aggressive pitch up attitude to level off at 6;500 feet. We became immediately task saturated with a congested frequency we were not able to query in time. I could have requested my FO query Approach about the altitude restriction; and I could have requested to slow early and decreased my descent rate. In the future; I will take the initiative exercise my authority in having my FO's query about instructions given by ATC to insure no miscommunications; and prevent task saturation; which will allow greater focus of the whole environment.
Second reporter narrative
We were inbound on ALLEE3 arrival into DEN. The entire arrival was turbulent. As we passed the fix DOGGG at 11;000; speed 210; approach had us descend and maintain 8;000. We read back clearance and began the descent. During the arrival the Controller announced LLWS and multiple aircraft were going around. The Controller asked another flight to speed up to 150; they said they were unable due to two previous go arounds. The frequency was congested. The Controller stated she was trying to space out all aircraft to get onto the approach. We were told to descend and maintain 6;500. I read back the altitude clearance; 6;500; and there was no response or correction. We had talked about what we were going to do to mitigate the LLWS/ go around threat. We confirmed the altitude with one another. The atmosphere was hazy. I was trying to divide attention between inside the cockpit and outside to maintain airport visual. During the descent; the Controller gave us a turn to heading 270 and a speed of 190. I read back the clearance. The winds were gusty; our ground track during the turn was wide. During a break in the frequency I reported field in sight. The Controller came back and did not clear us for visual but to standby and wait for more instruction. Within seconds the Controller came back to tell us they had gotten a low altitude warning on us. I looked down to see that the autopilot did not capture the altitude. 'Terrain Terrain' went off once; and the speed shaker went off momentarily. Immediately my Captain disconnected autopilot; stowed the flight spoilers; and increased thrust. I was watching the altitude increase. I got on the frequency and told the Controller once we were at 7;000. She came back and said she had cleared us for 7;500. She told everyone on frequency to watch their altitudes. As we got to 7;500 the Controller told us to turn to heading 325 and that we were cleared for the visual 35R. She gave us a speed of 150 to FRONZ. My Captain was hand flying and by this time we had slightly overshot the approach course. He turned to re-intercept the approach course. I cleaned the box; got the aircraft configured and switched to Tower. At FRONZ we got a wind shear caution. Initially my Captain began to climb and I was ready to readout trends but quickly we realized it was caution. As we realized that; the wind shear had gone away. Tower had advised us that we could expect 30 knots loss of airspeed 150 feet above touchdown. Tower had cleared us to land and we were able to safely land. I believe the cause of this event was due to the Controller giving us a low altitude of 6;500 and not receiving a correction during readback. The LLWS; reports of go arounds; and trying to get slow while descending created task saturation. Autopilot not capturing the 6;500 altitude was another contributing factor. To avoid recurrence of this event I will verify altitudes when given unusual ones by ATC. I will also watch the autopilot more diligently regardless of PF/PM. I will query to avoid confusion. I will be more cognizant of making sure the aircraft is always in a safe configuration.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.